ENGINEERING PRODUCTIVITY KIT
Power transmission

Gearbox-coupling package offers greater
stiffness

Charles J. Murray, Senior Regional
Editor

Chicago, IL--In servo applications, a machine
can never be too stiff. There, stiffness is one of the
few universal "goods," enabling better dynamic
performance and faster response.

Unfortunately, not every gearbox-coupling combination
provides the stiffness needed to optimize servo performance.
In many cases, engineers must dampen the machinery and
"de-tune" the servo performance, just to accommodate
the lack of stiffness in the gear reducer and coupling.

A new gearbox-coupling package from Gam Gear, however,
solves that problem. Known as I.M.P.A.C.T. (Integrated
Modular Planetary And Coupling Technology), it is shorter
and stiffer than conventional planetary gearboxes and
couplings. In essence, the I.M.P.A.C.T. is like a coupling
with a planetary gearbox at its core. It consists of
a corrugated metal bellows coupling on its input side,
a planetary gearbox in the middle, and a corrugated
metal bellows coupling on its output side.

The compactness and stiffness resulting from the design
are said to be ideal for dynamic applications, such
as packaging, stamping, and material handling machinery.
"It creates a much stiffer system," notes
Craig Van den Avont, general manager for Gam Gear. "With
this coupling, you can increase the gains on your servo
system and get much tighter system response."

The key to the I.M.P.A.C.T's compactness is its unusual
mounting of the coupling directly onto the gearbox's
planet carrier. Engineers from Jakob Antriebstechnik
in Germany, designers of the new system, accomplished
that by boring a hole in the center of the planet carrier
and applying a unique connection concept.

The concept involves three main parts: a split-sleeve
shaft that is integral to the coupling; a conical bushing;
and a bolt that connects the planet carrier to the coupling.
During assembly, the coupling shaft goes into the bored
hole. The bolt fits within the shaft and is threaded
through the bushing. As the bolt is tightened, it draws
the wide end of the cone into the split-sleeve shaft,
creating an interference fit between the shaft and the
bushing. This interference fit serves as the connection
between the planet carrier and the coupling, allowing
the two to rotate as a single entity.

The conical bushing design reportedly generates twice
as much clamping force as conventional radial clamping
hubs. It clamps at about 4.5 times the torque capacity
of the coupling, so in a worst case scenario the coupling
will break before it slips.

By employing the conical bushing connection, instead
of a side-mounted bolt, as most conventional couplings
would, the new design eliminates any balancing problems
caused by side-loading. It also simplifies maintenance.
The reason: The conical bushing configuration enables
users to easily remove the coupling. In contrast, conventional
couplings require that users loosen a screw and open
the gearbox housing.

For design engineers, the overall advantages of the
I.M.P.A.C.T. are numerous. The most obvious of those
is simplicity. Design engineers no longer need to select
input and output couplings when they add a gearbox to
a piece of machinery, since this design already includes
it. Nor do they need to add a bell housing, since that,
too, is included in the I.M.P.A.C.T. package.

On the input side, the I.M.P.A.C.T. coupling accommodates
thermal expansion and compensates for misalignment.
In contrast, many couplings don't allow for thermal
growth of the motor, which can damage motor bearings
or encoders. By using a metal bellows coupling on the
input side, however, the I.M.P.A.C.T. design allows
for thermal expansion of the motor shaft.

Most important, the new design reduces the length of
the gearbox-coupling package, improves torsional stiffness,
and reduces inertia. By eliminating the conventional
output shaft, engineers say they've cut 20-45 mm of
the typical length of the gearbox and coupling. The
improved compactness and elimination of the shaft have,
in turn, resulted in much greater stiffness.

Those advantages are expected to be critical for users,
particularly in servo applications. "Whether it's
printing, packaging or other applications, servo systems
need to make very dynamic, quick moves," Van Den
Avont says. "Any time you can improve the stiffness
of a servo system it's an advantage, because it allows
you to more effectively do those dynamic applications."

Other Applications

Composite wear rings improve pump performance

Charles J. Murray, Senior Regional
Editor

Kulpsville, PA--By employing rotating impeller
parts made from a composite, a south Texas refinery
has reduced maintenance costs and improved the mean
time between failure for its pumps.

The refinery, which was experiencing frequent catastrophic
failures of its boiler feed water pumps, was paying
out $15,000 per pump for repairs at six week intervals.
To solve the problem, the refinery replaced stainless
steel wear rings on the pumps with the composite.

The composite, made by Greene, Tweed & Co. and
known as WR525, consists of carbon fiber and polyetheretherketone
(PEEK) resin. It serves more effectively in the wear
rings because its lubricity makes it non-seizing and
non-galling. Those characteristics were critical for
the refinery application, where occasional cavitation
caused galling of the wear rings.

One of the key reasons for the material's performance
in high temperature, high pressure applications is its
thermal expansion coefficient, engineers say. Because
it has a different thermal expansion coefficient than
steel, it expands at a slower rate. As a result, when
the heat of the application increases, the WR525 wear
ring grows tighter around the shaft. This makes it almost
impossible for the pump to seize. It also reduces pump
vibration and improves pump efficiencies.

Greene, Tweed & Co. engineers say that the material's
performance in such applications is important to a rapidly
growing user community. "A few years ago, our customers
were happy to get one year intervals between maintenance,"
notes Jonathon Pledger of Greene, Tweed & Co. "Today,
they want five year intervals."

In the Texas refinery application, WR525 has increased
the mean time between failure and reduced the average
part repair cost from $15,000 to $3,000, Pledger says.

Nickel-based alloy stops galling in
food processing machinery

Charles J. Murray, Senior Regional
Editor

Waukesha, WI--Using a nickel-based alloy,
engineers may now have a more potent means to deal with
the problem of galling in food processing machinery.

The alloy, known as Waukesha 88, is mostly nickel,
but also includes tin, iron, bismuth and chromium. It
solves the problem of galling of stainless steel materials,
which are often used to combat corrosion-caused processing
of aggressive foods and pharmaceuticals.

Although it may seem innocuous at first glance, galling
of stainless steels can cause enormous problems for
food processors. The worst problems occur when stainless
steel surfaces rub against one another, potentially
releasing tiny steel particles into the food or drug
that's being processed. The problem is even more pronounced
in food processing, where the use of lubricants is often
not permitted, and galling is therefore increased.

Waukesha 88 solves that problem because it offers a
combination of properties. Like stainless steel, it
is corrosion resistant, making it ideal for use in the
processing of aggressive foods and pharmaceuticals.
Unlike stainless steels, however, Waukesha 88 also incorporates
bismuth, which provides an anti-galling effect. That,
in turn, enables its use in dynamic applications where
galling can be a problem.

As a result, some engineers now employ Waukesha 88
in rotors for positive displacement pumps, in bushings
for pump shafts, and as plungers and formplates for
meat patty machines. The alloy was recently used as
a piston material in multi-station filling machines,
which fill cans and bottles with foods ranging from
baby meals to ketchup.

Engineers say that the key is to use the material as
one of two mating parts, because cost often prohibits
its use in large shafts or housings. In the filling
machines, for example, engineers used it as a piston
but did not apply it to the mating part--a large cylindrical
casting.

"It's more costly than stainless steels,"
notes Tom Kerwin of Waukesha Foundry. "But it lasts
longer and solves the galling problems, so it makes
sense in critical applications like food processing."

Spring design made easy

When designing a wave spring into an application, it
can be time consuming and difficult to choose variables
affecting the design. The engineer normally starts by
defining the loads, work heights and free height. Next,
he or she establishes the cavity that the spring must
operate in.

Focus on Fundamentals consists of 45-minute on-line classes that cover a host of technologies. You learn without leaving the comfort of your desk. All classes are taught by subject-matter experts and all are archived. So if you can't attend live, attend at your convenience.